A standardization organization 3GPP (3rd Generation Partnership Project) is developing a radio access network technique, which is called UTRAN LTE (Universal Terrestrial Radio Access Network Long Term Evolution), for the next-generation mobile communication system. For the core-network, SAE (System Architecture Evolution) is being developed.
The LTE is being developed with an expectation of realizing the functions of a conventional radio base station (Node-B) and a radio base station controller (RNC: Radio Network Controller) in a single node, which is called eNB (evolved Node B).
FIG. 10 is a diagram illustrating a configuration example of the IMT-2000 LTE/SAE system based on the above development. The system includes a UE (User Equipment) 1001, an LTE 1002, an SAE 1003, and an IP service network 1004. The LTE 1002 is called E-UTRAN, in which eNBs 1002-1 are disposed. In the SAE 1003, an MME (Mobility Management Entity) 1003-1, a Serving Gateway 1003-2, an HSS (Home Subscriber Server) 1003-3, a PDN (Packet Data Network) Gateway 1003-4, and a PCRF (Policy Charging Rule Function) 1003-5 are disposed. The IP service network 1004 is a function unit that provides application services (multimedia communication, packet communication), and includes various server groups that provide application services such as IMS (IP Multimedia Service) and PSS (Packet Switching Service).
For the IMT-2000 LTE/SAE system, the interface between eNBs is called an X2 interface, and the interface between an eNB and an MME/SAE is called an S1-AP interface. The protocol stacks of the X2 interface and the S1-AP interface are illustrated in FIG. 11.
A mobile phone or a PHS (Personal Handyphone System) with mobile communication that adopts the cell system cannot continue communication when the signal from a base station weakens as the terminal moves to the cell boundary or due to other reasons. Therefore, when the signal weakens or before the signal weakens, a procedure called handover (H/O, H. O.) is performed to switch the connection to a base station of another cell with a strong signal.
For the handover in the IMT2000 LTE/SAE system, there are two types, namely, (1) handover using the X2 interface and (2) handover using the S1-AP interface.
The handover using the X2 interface is a handover procedure performed when the X2 interface between a handover-source eNB 1002-1 (Source eNB) and a handover-destination eNB 1002-1 (Target eNB) is established. The MME 1003-1 is not involved in the handover control. The operation sequence of the handover using the X2 interface is illustrated in FIG. 12.
The handover using the S1-AP interface is a handover procedure performed when the X2 interface between a Source eNB 1002-1 and a Target eNB 1002-1 are not established. This procedure is performed in different ways depending on whether the Source eNB 1002-1 and the Target eNB 1002-1 belong to the same MME 1003-1 or to different MMEs 1003-1. The operation sequence of the handover using the S1-AP interface (where the Source eNB 1002-1 and the Target eNB 1002-1 belong to the same MME 1003-1) is illustrated in FIG. 13.
Respective procedures in using the handover procedure of an S1-AP message during the handover between eNBs 1002-1 that do not go beyond the MME 1003-1 are described below.
FIG. 14 is an operation sequence diagram of the HANDOVER PREPARATION procedure (S1-AP).
In the handover between eNBs 1002-1 that do not go beyond the MME 1003-1, first, the Source eNB 1002-1 receives a handover request (RRC: MEASUREMENT REPORT) from the UE 1001. Then, a check is performed, from Target eNB information in the handover request, as to whether the X2 interface between the Source eNB and the Target eNB has been activated. When it is determined that the X2 interface has not been activated yet as a result of the check, the Source eNB 1002-1 sends a handover request (S1-AP: HANDOVER REQUIRED) to the MME 1003-1. (FIG. 14, S1401). The response (success or failure) is determined depending on the success (S1402) or failure (S1403) of the HANDOVER RESOURCE ALLOCATION procedure illustrated in FIG. 15.
FIG. 15 is an operation diagram of the HANDOVER RESOURCE ALLOCATION procedure (S1-AP).
First, the MME 1003-1 receives an S1-AP: HANDOVER REQUIRED (FIG. 14, S1401). Then, the MME 1003-1 checks, from I.E (information of the Target eNB and so on) in the HANDOVER REQUIRED, whether the Target eNB 1002-1 is not beyond the MME 1003-1 (the Target eNB 1002-1 exists under the MME) or the Target eNB 1002-1 is beyond the MME 1003-1 (the Target eNB 1002-1 does not exist under the MME). When it is determined that the Target eNB 1002-1 is not beyond the MME 1003-1 as a result of the check, the MME 1003-1 sends a handover request (d2S1-AP: HANDOVER REQUEST) to the Target eNB 1002-1 (FIG. 15, S1501).
Upon receiving the S1-AP: HANDOVER REQUEST, the Target eNB 1002-1 performs the resource capture at the Target eNB side for the handover, and the like, and returns the response (success (S1502) or failure (S1503)) on the request to the MME 1003-1. Then the MME 1003-1 returns, based on the response result on the request, the response (FIG. 14, S1402 or S1403) in the HANDOVER PREPARATION procedure to the Source eNB 1002-1.
FIG. 16 is an operation sequence diagram of the HANDOVER NOTIFICATION procedure (S1-AP).
The Target eNB 1002-1 gives, to the MME 1003-1, a notification that the UE 1001 has been connected to the cell of the Target eNB 1002-1, and the S1-AP handover has been completed (FIG. 16, S1601).
FIG. 17 is an operation sequence diagram of the X2 setup procedure (X2-IF).
In this procedure, data exchange at the application level that is required between the eNBs 1002-1 for the accurate interoperation is performed. First, an X2 SETUP REQUEST is transmitted from the transmission-source eNB 1002-1 to the transmission-destination eNB 1002-1 (S1701). Then, a success response X2 SETUP RESPONSE (S1702) or a failure response X2 SETUP FAILURE (S1703) is returned from the transmission-destination eNB 1002-1 to the transmission-source eNB 1002-1.
FIG. 18 is an operation sequence diagram of the S1 setup procedure (S1-AP).
In this procedure, data exchange at the application level that is required for the eNB 1002-1 and the MME 1003-1 for realizing the appropriate communication on the S1-AP interface is performed. First, an S1 SETUP REQUEST is transmitted from the transmission-source eNB 1002-1 to the transmission-destination MME 1003-1 (S1801). Then, a success response S1 SETUP RESPONSE (S1802) or a failure response S1 SETUP FAILURE (S1803) is returned from the transmission-destination MME 1003-1 to the transmission-source eNB 1002-1.
The message formats used in the S1-AP handover and X2 setup procedure (X2-IF) are illustrated in FIG. 19 through FIG. 25. FIG. 19 is the format of the HANDOVER REQUIRED (S1-AP) message. FIG. 20 is the format of the HANDOVER REQUEST (S1-AP) message. FIG. 21 is the format of the HANDOVER COMMAND (S1-AP) message. FIG. 22 is the format of the S1-AP I.E Criticality Diagnostics message. FIG. 23 is the format of the X2 SETUP REQUEST (X2-IF) message. FIG. 24 is the format of the X2 SETUP RESPONSE (X2-IF) message. FIG. 25 is the format of the S1 SETUP REQUEST (S1-AP) message.
In the handover between eNBs 1002-1, in order to prevent the data loss, the Source eNB 1002-1 may perform Data forwarding of the user data to the Target eNB 1002-1.
FIG. 26 illustrates the data forwarding with the handover using the X2 interface.
It is assumed that before the handover, data d1, d2, d3, d4, d5, d6 were sent from a Serving Gateway 1003-2 to a Source eNB 1002-1. Here, if the handover (H.O.) occurs in a state in which the data d1, d2, d3 have reached the UE 1001, the Source eNB 1002-1 performs the Data forwarding, to the Target eNB 1002-1 by means of the X2 interface, of the data d4, d5, d6 that have not been transmitted to the UE 1001. During the handover procedure, the path switch from the Source eNB 1002-1 to the Target eNB 1002-1 is performed, and the subsequent data d7, d8, d9 are transmitted from the Serving Gateway 1003-2 to the Target eNB 1002-1.
FIG. 27 illustrates the data forwarding with the handover using the S1 interface.
It is assumed that before the handover, the data d1, d2, d3, d4, d5, d6 were sent from the Serving Gateway 1003-2 to the Source eNB 1002-1. Here, if the handover occurs in the state in which the data d1, d2, d3 have reached the UE 1001, the Sour eNB 1002-1 performs the Data forwarding, to the Target eNB 1002-1 via the Serving Gateway 1003-2, of the data d4, d5, d6 that have not been transmitted to the UE 1001. During the handover procedure, the path switch from the Source eNB 1002-1 to the Target eNB 1002-1 is performed, and the subsequent data d7, d8, d9 are transmitted from the Serving Gateway 1003-2 to the Target eNB 1002-1.
Note that related art may be described, for example, in Japanese Laid-open Patent Publication No. 2008-103865 and Japanese Laid-open Patent Publication No. 2008-227772.
For the handover between LTEs 1002 illustrated in FIG. 10, the 3GPP defines that when it is made via the SAE 1003, the S1-AP interface is used, and when it is not made via the SAE 1003, the X2 interface is used. The S1-AP interface corresponds to the inter-station link between the eNB 1002-1 and the SAE 1003, and the X2 interface corresponds to the inter-station link between the eNBs 1002-1.
In the conventional art, the inter-station link is set up or cancelled with the addition or reduction of an eNB 1002-1, and is set up to all adjacent radio base stations.
However, since resources that are required for the setting of the inter-station link are limited, if the inter-station link is set up for all eNBs 1002-1, there is a risk of resource shortage. Here, the number of adjacent eNBs 1002-1 may increase due to the operation of a Home eNB (home base station) and the like.
In addition, since the inter-station link is set on SCTP (Stream Control Transmission Protocol), the exchange of a message with an adjacent eNB 1002-1 takes place due to the HEATBEAT function of the SCTP, even in a state under which no communication is performed. As a result, the processing load may increase.
Therefore, in the IMT-2000 LTE/SAE system, it is expected that setting the X2 interface (inter-station link) for all adjacent eNB 1002-1 at the time with resume or addition is not practical. However, if many handovers occur while X2 interface is not set between eNBs 1002-1, the handover using the S1-AP interface takes place many times. As a result, the load for SAE may increase and available line band and resources for the S1-AP interface may be reduced, which is expected to cause the degradation of the handover service.